Ecosystem development in short-term postglacial chronosequences: N and P limitation in glacier forelands from Santa Inés Island, Magellan Strait

Abstract

Glacier foreland moraines provide an ideal model to examine the patterns of ecosystem development and the evolution of nitrogen and phosphorous limitation over successional time. In this paper, we focus on a 400-year soil chronosequence in the glacier forelands of Santa Ines Island in the Magellan Strait, southern Chile by examining forest development on phosphorus (P)-poor substrates in a uniquely unpolluted region of the world. Results show a steady increase in tree basal area and a humped trend in tree species richness over four centuries of stand development. The increase in basal area suggests that the late successional tree species were more efficient nutrient users than earlier successional ones. Total contents of carbon (C) and nitrogen (N) in soils increased during the chronosequence, reaching an asymptote in late succession. The net increases in soil C : N, C : P and N : P ratios observed over successional time suggest that nutrient limitation is maximal in 400-year-old substrates. Foliar C : N and C : P ratios also increased over time to reach an asymptote in old-growth stages, following soil stoichiometric relationships; however the foliar N-to-P ratio remained constant throughout the chronosequence. Biological N fixation was greater in early postglacial succession, associated with the presence of the symbiotic N-fixer Gunnera magellanica. Declining trends of δ15N in surface soils through the 400-year chronosequence are evidence of decreasing N losses in old-growth forests. In synthesis, glacier foreland chronosequences at this high South American latitude provide evidence for increasing efficiency of N and P use in the ecosystem, with the replacement of shade-intolerant pioneers by more efficient, shade-tolerant tree species. This pattern of ecosystem development produces a constant foliar N : P ratio, regardless of variation in soil N-to-P ratio over four centuries.